There are conventionally known processors that can perform accurate facing operations with no need of centering work on a hollow cylindrical workpiece. One example of the processors of the sort as stated just earlier is disclosed in Japanese Patent Laid-Open No. 2002-337012, which is envisaged turning a workpiece of material heat-treated to be highly hardened. The prior processor is composed of a machine spindle supported for rotation on a headstock fastened on a machine bed, a magnet chuck mounted on a lengthwise end of the machine spindle in a way revolving coaxially with the machine spindle, a loose headstock placed on the same rotational axis with headstock, and a three-jaw chuck-mounted on a lengthwise end of the loose headstock in a way revolving coaxially with the loose headstock. The three-jaw chuck or self-centering chuck is to transfer the workpiece held in the chuck to the magnet chuck with keeping the rotational axis of the work in alignment with the rotational axis of the magnet chuck, thereby performing the centering operation.
The processor constructed as stated earlier, nevertheless, has need of a carriage to carry the loose headstock thereon, and therefore could not get out of becoming bulky in construction. Moreover, the workpiece held in the three-jaw chuck, as deformed easily as shown in FIG. 9, is unfair disadvantageous to the centering operation and therefore poses an issue of causing variations in centering from one work to another.
As shown in, for example, Japanese Patent Laid-Open No. H10-43985, an automatic centering process for machining more massive parts has been developed which helps a circular workpiece make automatic centering with accuracy as well as with a very short time on a machine tool. The level of accuracy in centering to be varied depending on the property of workpiece can be selectively determined using an inspection means. With the prior automatic centering process recited earlier, the centering operation is performed with the combination of radially retractable driver units and an inspection unit to detect off-center deflection. The radially retractable driver units are each arranged at a point of three o'clock and another point of from six o'clock to eight o'clock around a circular surface of a workpiece held in a magnet chuck. At a centering mode, the magnet chuck switches to a weaker level in magnetic force. Simultaneously with this, the workpiece starts revolving while the driver units move center-ward and the inspection unit shifts towards a reference surface for measuring the accuracy of centering. After the accuracy of centering comes into a prescribed range that can be tolerated by the inspection unit, a signal to cease the centering work is issued to retract radially the driver units into their home positions where the magnet chuck switches automatically to a stronger level and the machine is turns into a cutting mode at the same time.
The automatic centering process constructed as stated earlier, however, involves for centering operation activating the driver units set at preselected two points separately from one another while detecting the accuracy of centering by the inspection unit. With the prior automatic centering process, thus, there are practical issues that the machine tool has to be made bulky in construction and also lacks in universality for a variety of workpiece size.
As disclosed in, for example, Japanese Patent Laid-Open No. 2002-260293, there is known a centering process to bring the center of a circular plate into coincident relation with the center of a turntable. With this prior centering process, a turntable onboard a circular plate is moved straight by a linear guide mechanism until the outward circular edge of the circular plate comes into abutment against stoppers, thereby getting the center of the circular plate matching the rotational center of the turntable. With the centering device to carry out the centering process as stated earlier, the stoppers are placed in a relation spaced away from one another with an interval less than the diameter of the turntable, so that the circular plate at the first place comes into engagement at the outward circular edge thereof with any one of the stoppers, followed by coming into abutment against the other stopper to make the off-center correction, thereby performing the accurate centering.
The prior centering device constructed as stated earlier needs to set the stoppers with high accuracy in their locations and also lacks in universality for wide variations in circular plate size.
A centering apparatus for a circular plate with no need of rotating a turntable is known conventionally as disclosed in, for example, Japanese Patent Laid-Open No. 2003-157589. The prior centering apparatus for the circular plate as recited earlier is comprised of a turntable to carry a circular plate thereon, and arms with pads at their opposite distal ends laid in diametric opposition with respect to the turntable in a way pivoted at their centers for rocking motion about their fulcrums. As the arms shift towards the center of the turntable, the pads at the opposite ends of the arms come into contact with the outward circular edge of the turntable to follow the contour of the turntable.
However, the centering apparatus for the circular plate in which the four pads are arranged to embrace the circular plate would have a margin of error because the four pads doing their works on the circular plate could vary from one pad to another pad in working position. Moreover, the machine tool has to be made bulky in construction.